1. Field of the Invention
The present invention relates to a composition for resin magnets, a magnetic member using the same, and a process for producing said magnetic member. More particularly, the present invention pertains to a composition for resin magnets which is well suited for producing a magnetic member having a strong magnetic force, a magnetic member such as magnet rollers and magnet pieces which is produced by the use of the aforesaid composition, and a process for efficiently producing the aforesaid magnetic member.
2. Description of the Related Arts
There is known, in the field of electrophographic equipment and electrostatic recording equipment such as copying machinery and printers, a method for visualizing an electrostatic latent image by locating a magnet roller which is formed with a composition for a resin magnet in a rotating sleeve as a developing roller for the purpose of visualizing an electrostatic latent image on a latent image holding body such as a photosensitive drum and by supplying the surface of the latent image holding body with a magnetic developer (toner) through a so-called jumping phenomenon that allows the toner which is held on the surface of the sleeve to jump over onto the latent image holding body by means of the magnetic characteristics of said magnet roller.
The above-mentioned magnet roller has heretofore been produced by injection molding or extrusion molding of a composition for resin magnets in which magnetic powders are mixed with a thermoplastic resin binder, by the use of a mold which generates a magnetic field on the circumference of a cavity thereof to mold the composition into the form of roller and magnetize to desired magnetic force characteristics.
There is a tendency that a magnet roller is called upon to be equipped with further intricate magnetic force patterns with the recent advance of electrophographic equipment and the like. However, there has been a limitation on the magnetic force patterns designable with conventional magnet rollers, thereby making it impossible to sufficiently respond to the above-mentioned requirement.
In such circumstances in order to enhance the degree of freedom for the magnetic force pattern of a magnet roller, there has recently been constituted a desired magnetic force pattern by molding the above-mentioned composition for resin magnets into a plurality of magnet pieces that are magnetized to have magnetic poles according to the desired magnetic force pattern and sticking the resultant magnet pieces onto the surroundings of the shaft of the magnet roller.
As such composition for resin magnets for the purpose of producing a magnetic member such as magnet rollers and magnet pieces, there has heretofore been employed a composition of a resin binder comprising, as a principal component, a polyamide resin such as polyamide-6 and polyamide-12, or a thermoplastic resin such as propylene resin, vinyl chloride resin and ethylene/ethyl acrylate copolymer resin (EEA), and magnetic powders of ferrite, a rare earth metal magnet or the like that are mixed with and dispersed in said resin binder.
In addition, there are principally adopted an injection molding method in a magnetic field and an extrusion molding method in a magnetic field as a method for molding the composition into a magnetic member. In these cases, the injection molding method enables to produce a magnetic member having enhanced magnetic characteristics, since said method injects molten composition for resin magnets in a state of applying a magnetic field to a mold so as to orientate the magnetic powders in said composition for resin magnets in accordance with the resultant magnetic field, whereby said composition for resin magnets is allowed to cool in the mold to reach such a level of a viscosity that the foregoing orientation state is preserved.
On the contrary, conventional extrusion molding methods have involved the problem that nothing but a magnetic member with poor magnetic characteristics can be produced as compared with the above-mentioned injection molding method.
FIG. 1 is an explanatory illustration showing one example of extrusion molding method in a magnetic field, wherein a composition for resin magnets is heat molten in an extruder 1 and is extruded in a fluid state with a screw 2 into a magnetic field formed by a magnetic field-applied member 3 so as to form a molded product.
In such extrusion molding, the composition for resin magnets can be regarded as being a Bingham fluid at the temperature at the time of the extrusion molding, and the flow velocity is highest in the middle of the tube cross-section in a spinneret for the extruder and is lowest in the vicinity of the wall of the tube cross-section. As the result, since a magnetic field is applied in such a fluidity state, the magnetic powders that are to be orientated in the direction of the applied magnetic field are disturbed by the flow of the composition, whereby it is made difficult to obtain a magnetic member having a strong magnetic force. For instance, it is extremely difficult to obtain a magnetic molded-product having a surficial magnetic flux density exceeding 1000 Gauss with regard to magnetic molded products that are produced in the working examples of this specification hereinafter described unless the molding is carried out under specific condition.
In order to obtain a magnetic member having a desired magnetic force, an ideal flow pattern is considered to be a constant flow velocity of the composition in the same cross section of a spinneret. It is thought in this case that the magnetic member can be imparted with a strong magnetic force, since the orientation of the magnetic powders is totally unaffected by the friction and flow-velocity field due to the flow in the spinneret.
There has heretofore been taken, for instance, a countermeasure of enhancing the smoothness of the inside surface of a spinneret for the purpose of bringing the flow-velocity field at the time of extrusion molding to an ideal flow-velocity field. The above-mentioned countermeasure, however, has involved the problem that although effective at an extremely low extrusion velocity, it results in failure to exhibit the effect at a high extrusion velocity.
The extrusion molding method in a magnetic field, though being involved with such problem, is advantageous as compared with the injection molding method in that said extrusion molding method can shorten the molding tact and processing time because of continuous processing made possible, it can simplify and miniaturize the mold thus lowering the production cost thereof and at the same time, enables the production of a magnet roller excellent in uniformity of magnetic force such as lessened difference in the surficial magnetic force along the longitudinal direction. Under such circumstances, it has eagerly been desired to develop a technique enabling the production of a magnetic member such as magnet rollers and magnet pieces which has a strong magnetic force by means of an extrusion molding method in a magnetic field.
In such circumstances, a general object of the present invention is to provide a composition for the production of a resin magnet having a strong magnetic force, a magnetic member having a strong magnetic force obtainable by the use of the above-mentioned composition, and a process for efficiently producing said magnetic member.
In order to attain the above-mentioned object, intensive research and investigation were accumulated by the present inventors. As a result, attention has been paid to the fact that the composition for resin magnets can be regarded as being a Bingham fluid at the temperature at the time of extrusion molding, and the flow of said composition in a spinneret is preferably brought to an ideal state as close as possible by controlling its viscosity characteristics. In this connection, the following findings and information were obtained.
It has been found that the flow of a composition for resin magnets in a spinneret can be brought close to an ideal state, provided that xcex7R/xcex7O and xcex3 together satisfy a definite relationship for xcex7O measured within a specific range of temperature region and xcex3 being within a specific value, wherein with regard to the viscosity characteristics in molten state of said composition for resin magnets, xcex7R represents shear stress (dyne/cm2), xcex7O denotes yield stress {shear stress (dyne/cm2) when shear rate by extrapolation is zero} and xcex3 stands for shear rate (Sxe2x88x921), whereby said composition can conform to the purpose of use as a molding material for magnetic members. Moreover it has been found that a magnetic member having a strong magnetic force can easily be obtained by subjecting said composition to the extrusion molding in a magnetic field under a specific temperature and pressure. The present invention has been accomplished by the foregoing findings and information.
Specifically the present invention provides:
(1) a composition for resin magnets which comprises a thermoplastic resin and magnetic powders and which, when determined in a heated molten state thereof, has viscosity characteristics characterized in that the almost linear line which is obtained by taking xcex7R/xcex7O as ordinate and xcex3 as abscissa has a slope of at most 0.02 within a specific temperature range in which xcex7O falls within the range of from 2.0xc3x97106 to 1.0xc3x97107 dyne/cm2, wherein xcex7R denotes shear stress (dyne/cm2), xcex7O denotes yield stress {shear stress (dyne/cm2) when shear rate by extrapolation is zero} and xcex3 denotes shear rate (Sxe2x88x921);
(2) a magnetic member which is produced by the use of the composition for resin magnets as set forth in the preceding item (1); and
(3) a process for producing a magnetic member by subjecting the composition for resin magnets as set forth in the preceding item (1) to extrusion molding in a magnetic field at a temperature in the range of 80 to 300xc2x0 C. at a pressure in the range of 10 to 300 kgf/cm2.